Pelle said emergency crews were worried about a Lyons Fire Department crew stuck on a mountainside near a washed-out road. Hypothermia from the cold, wet conditions was of particular concern, he said.
As of Thursday evening, authorities in Boulder had not been able to make contact with the Lyons Fire Department to confirm that the firefighters had been rescued, Herring said.
As dawn broke over the region, Pelle said he was "amassing a large-scale effort" to rescue those who are trapped, reach those who are stranded and deliver much needed aid to places like Lyons, where floodwater overtopped several dams protecting the town.
Hickenlooper said later in the day that two National Guard Black Hawk helicopter search-and-rescue crews were sent to Boulder County, along with three swiftwater rescue teams and emergency managers. One helicopter and emergency managers were dispatched to Larimer County.
Hickenlooper said the state didn't immediately need outside help. The problem was reaching the most critical areas.
Rescuers have been frustrated by debris, impassable roads and mudslides, Pelle said.
"This is not your ordinary disaster," Pelle said. "All the preparation in the world, all the want-to in the world, can't put people up those canyons while debris and walls of water are coming down."
The rain started falling in earnest about 6 p.m. Wednesday and continued into Thursday, sometimes at the rate of about an inch an hour, according to radar estimates. That added up to about 6 to 7 inches of total rainfall.
"The rain, it almost feels like hail, the drops are so thick," University of Colorado Boulder student Ryan Colla told CNN affiliate KUSA. "It just keeps coming and coming, and when you think it's going to subside, it starts to rain down harder."
The sudden influx of water turned Boulder Creek -- which runs through the campus and other parts of the city bearing its name -- into a high, fast, muddy and dangerous torrent, Colla told the station.
"It freaked us out," he said.
At its peak, Boulder Creek was flowing at 16 times its normal rate for this time of year, city spokeswoman Sara Huntley said.
But that was not the only stream causing trouble, Pelle said. Unlike the last devastating flood in Boulder in 1969, this storm caused virtually every waterway in the area to overflow, he said.
Water rushed through Aurora, east of Denver, swirling and breaking like an ocean hitting a beach. CNN affiliate KCNC captured video of one person stumbling dangerously while trying to cross an Aurora street and finally struggling at the edge of the water. Three onlookers pulled that person to safety.
In Estes Park in Laramie County, KCNC video showed the Big Thompson River rushing above both sides of a bridge and flooding several small businesses.
And rain is likely to continue to pose problems, CNN meteorologist Chad Myers said.
"It could rain again all night," he said.
Dams threatened, roads washed away